1. Technical Field
The present invention relates to a device for pipetting a liquid, comprising a pipette body enclosing a pipette volume in which a fluid medium can be placed and in which an actuator that controllably displaces the medium in the pipette volume is provided, as well as a pipette tip that can be connected in a fluid-tight manner to the pipette volume.
2. Description of the Art
For example, in studying the findings of diagnostic examinations or routine physical examinations, modern medicine is increasingly relying on quantitative analysis of relevant substances in body fluids. The number of to-be-examined substances is continuously growing as is test frequency. Increasing numbers of analyses with simultaneously decreasing costs requires, in particular, lowering the need for reactants, which are often quite expensive. Therefore, the trend in development is to focus on precise dosage of as small as possible amounts of liquid.
Used for this purpose is high-throughput screening, which permits studying the interaction of a drug, molecule or a cell with a great number of different substances in an economical manner. Defined volumes of corresponding solutions are brought to react in microtiter plates, in short MTP. In order to be able to work quickly and cost-effectively, the testing volumes are further miniaturized. In a 1536 microtiter plate, the volumes, after addition of all substances and reactants are less than 10 μl.
As the mixture ratio of the to-be-tested substances and the reactants may vary considerably, amounts of liquid in the μl range must be dosed precisely. Moreover, in order to avoid contamination effects, it is essential, in particular, that the smallest amounts are the first to be dispensed into the individual, separate dry reaction chambers, the so-called wells, on the microtiter plate.
If one and the same volume of the same liquid is to be distributed into a multiplicity of wells, piezoelectric (also see “nano-plotter” at http://www.qesim.de or “Genesis NPS” at http://www.tecan.de) or ink jet microdispenser systems (see D. Rose, “Microdispensing Technologies in Drug Discovery”, DDT, vol. 4, No. 9, 1999) can be utilized very effectively, which are able to “shoot” tiniest drops into the individual wells without coming into contact.
FIG. 2, shows a known drop dispensing system based on a state-of-the-art piezoelectric pump. The dispensing system comprises a microtiter plate MTP disposed on a housing G, over which a needle pipette N with a single pipette channel is provided in a three-dimensionally turnable manner. Provided in the housing G is an injection pump S which conveys a carrier fluid T to the needle pipette N via a fluid-filled tube L. The needle pipette N is provided with a piezo-pump PP through which the carrier fluid T can be made to start oscillating in the needle pipette N. An air bubble B separates a liquid sample P from the carrier fluid T in the pipette tip PS. In this manner, excited by pressure oscillations in the carrier fluid, tiniest drops of the sample liquid are shot into the individual wells W on the microtiter plate MTP.
A drawback is that due to functional determinations, a relatively large minimal sample volume of several μl has to be accommodated in the pipette tip. If a very small amount of sample liquid is to be dispensed only once, a major part of the, in some cases very expensive, sample material is wasted. Furthermore, dosing viscous liquids in this manner is difficult. Moreover, piezo-pumps are very expensive and have hitherto been built with only 8 parallel pumps. Also known are Cartesian Technologies' pipette arrangements with 96 pipettes, which, however, are extremely complex as each pipette channel requires a tip. The aforementioned methods are very vulnerable to disturbances as the dosage process can only be controlled to a limited extent.
If, however, a multiplicity of different samples need to be dispensed only once, for example, a MTP copied or converted, pipette devices according to the so-called displacement principle are still employed, which are provided with 96 or 384 parallel channels. FIGS. 3a and 3b each show a piston-driven pipette array. In FIG. 3a, only a single large piston K serves as the drive for the entire pipette array PA, e.g. in CyBio AG's devices, see “Cybi-Well 2000” at http://www.cybio-ag.com. The samples are sucked on or dispensed via an air cushion. As disposable plastic tips PS can be used, the risk of contamination by entrainment of sample remains is very small. Due to the elasticity of the relatively large air cushion, the minimal pipette volume is limited to approximately 1 μl. In FIG. 3b, every single pipette channel PK is provided with a separate piston, which is moved by a common drive A, for example as in Robbins Ltd.'s “Hydra”, at http://www.robsci.com. As, in this case, the samples can be manipulated almost without an air cushion, sub-μl volumes can be conveyed. However, instead of replaceable plastic tips PS, special, firmly held steel needles are employed, which require complicated rinsing following each pipetting procedure.
As none of the prior art pipetting systems permit checking how much sample liquid has actually been dispensed into each single well, it must always be ensured that the change in volume due to the piston movement is conveyed evenly and accurately into the pipette tips. Therefore, in all the prior art pipetting devices working in an array, the pipette tips are rigidly attached to the pipette head in which the piston is provided. Reusable tips are screwed fast onto the pipette head. On the other hand, disposable plastic tips are immobilely stuck into or clamped onto a receiving plate with corresponding connecting pieces.
In order to be able to guarantee that the desired amount of sample liquid is conveyed from each single pipette tip into the corresponding well, it must furthermore be ensured that the pipette tip or at least the amount of separated liquid conveyed from the pipette tip comes into contact with the surface of the well, if a method of drop discharge, as mentioned in the introduction hereto with reference to FIG. 2 is to be obviated. Fundamentally, precise operating X-Y-Z positioning devices for micropipette units relative to MTPs are known but problems crop up when the MTP curves or buckles three-dimensionally. Thus, under circumstances, not all the pipette tips of an array or the separated amounts of sample come into contact with the bottoms of the wells. Consequently, in some cases, no sample liquid is conveyed onto the MPT, but rather the sample remains hanging on the tip. Errors of this type occur spontaneously and can only be detected by visual control of the finished, pipetted MTP.
Robbins Ltd. (http://www.robsci.com) has discovered a special solution for avoiding the aforedescribed problem. The long, thin steel pipette needles set down on the bottom of the MTP with a thrust in such a manner that they bend with little elasticity. In this manner, it is ensured that all the tips really do come into contact with the bottom of the wells. If, on the other hand, disposable plastic tips are to be employed for pipetting, this method does not work, because the standard plastic tips are very rigid. If corresponding pressure were exerted on them, they would deform irreversibly but not bend.